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December 13, 2011


Internal Parasites in your horse

by DePaolo Equine Concepts

Horses share a symbiotic relationship with several varieties of internal parasites. In small numbers, this relationship is both normal and healthy. The parasites gain a host that enables them to mature and perpetuate their life cycle, while the horse obtains a measure of resistance to further parasitic infection. However, in large numbers, internal parasites pose a serious risk to a horse’s health. These parasites include ascarids (round worms), large and small strongyles (blood worms), tapeworms, bots, pinworms, stomach worms, neck threadworms and lungworms. Each parasite has its own unique infection pattern, health implications and treatment protocols. The sheer variety is dizzying and can make parasite management seem a daunting task.

In North America, four parasites pose the greatest risk to a mature horse’s health:

large and small strongyles


and bots.

Ascarids (round worms) are a concern in horses under the age of two, yet most foals develop immunity after about eighteen months. The two most prevalent parasites are large and small strongyles. In excessive quantities these worms can cause colic, weight loss, fever, poor coat, anemia, as well as threaten the health of arterial walls and blood vessels.

Symptoms of parasite infestation include:

  • Weight loss
  • Poor hair coat
  • Colic
  • Dermatitis
  • Chronic Tail rubbing
  • Respiratory Issues
  • Poor wound healing

Types of internal parasites include:

Ascarids (Round Worms): These are the largest of the parasites, growing to as long as one foot. They are most common in foals and many horses develop immunity by the age of eighteen months. These worms are linked to cold, fever, loss of appetite, decreased weight and susceptibility to pneumonia.

Large and Small Strongyles (Bloodworms): The two most prevalent worms affecting horses over the age of two in North America. Small strongyles are more difficult to eradicate because they encyst in stomach lining, where they can remain dormant for years before maturing and laying eggs. This also makes them immune to most dewormers until they emerge from they protective cysts. These parasites cause colic, weight loss, fever, anemia, and deterioration of the arterial walls and blood vessels.

Bots: Bot eggs (pictured below) laid by female bot flies on a horse’s head, legs or neck, are ingested and pass into an untreated horse’s manure where they hatch and develop into adult flies. These parasites are not worms, but are responsible for ulcers and perforations of the stomach wall.

Tapeworms: An increasing concern, especially for horses in the East and locations like Michigan and Wisconsin, these parasites can grow to four feet in length and are linked to increased colic – specifically spasmodic (gas-induced) colic.

Pinworms: Though fairly uncommon, pinworm eggs are sticky and can cling to stall walls and other surfaces. These parasites are can cause severe itching.

Stomach Worms: These parasites reside in the stomach where they lay eggs that are passed through to manure where they adhere to host maggots. When the maggots develop into flies and land on open wounds, worms are deposited that can cause irritation and impede healing.

Neck Threadworm: Transmitted by gnats, these parasites live in tendons and ligaments, and are associated with severe dermatitis.

Threadworm: A parasite that typically only affects foals, the threadworm can cause severe diarrhea and damage to intestinal lining. Horses typically achieve immunity after six months of age.

Lungworm: These parasites travel to the airways via the blood stream and can cause respiratory issues and an overall deterioration in health.

In short, Internal Parasites are a natural and healthy presence in horses at low levels.  Excessive parasite levels can cause serious health issues and must be reduced.  For a list of common de-worming medicines, please visit our blog post on Typical Parasite Treatments.

2 Comments Post a comment
  1. Jul 15 2012

    I recommend the use of DE.. Diatomaceuos-Earth.. as the most efficient, practical method of
    TOTAL ELIMINATION of ALL internal parasites…..


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  1. Typical parasite treatments for horses and how they work | DEC blog

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